1. Field of the Invention
The present invention relates to an agglomerated pigment, a process for producing the same, an aqueous pigment dispersion, and a water-based ink composition. Use of the novel agglomerated pigment according to the present invention readily gives an aqueous pigment dispersion in which the pigment is in a satisfactorily dispersed state and which after preparation undergoes little change in viscosity or pigment particle diameter during storage. This aqueous pigment dispersion can be advantageously used as a colorant for water-based inks such as ink-jet printer inks, writing utensil inks, etc.
2. Description of the Prior Art
Many investigations have recently been made on the utilization of pigments, in place of dyes, as a colorant for ink-jet printer inks, writing utensil inks, etc., because of the excellent fastness properties of pigments. In this application, it is important to stably disperse fine particles of a pigment into water since pigments are insoluble in water unlike dyes. A generally employed technique for improving the wettability of a pigment by water and thus preventing pigment sedimentation is to add to the pigment one or more dispersants selected from various surfactants, water-soluble resins, and the like and reduce the pigment into finer particles using a dispersing machine such as a sand grinder or a ball mill.
For example, JP-A-54-10023 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a water-based ink composition for writing utensils which comprises a pigment, a dispersant, and an aqueous medium, and in which the dispersant is a polymer having a hydrophilic part and an oleophilic part and the aqueous medium is a nonvolatile hydrophilic organic solvent. In JP-A-56-147871 is described a recording liquid comprising at least a pigment, a polymeric dispersant, and an aqueous medium containing a nonionic surfactant. In JP-A-56-155262 is reported a recording liquid and the like which have been improved in stability by employing a combination of a pigment and dye of similar colors. However, these ink compositions have problems such as flocculation of dispersed fine particles and changes in properties, e.g., viscosity. The prior art ink compositions thus have insufficient stability and are unsuitable for use in the thin tubes of writing utensils or in the minute nozzles of ink-jet printer heads, in which resolution is being heightened more and more in recent years.
Many proposals have been made on techniques for eliminating the drawbacks described above which comprise improving stability by modifying the chemical properties of a dispersant or subjecting a pigment to a surface treatment or the like. For example, an ink composition for ink-jet recording which employs a polymeric dispersant is reported in JP-A-4-85375. In JP-A-5-179183 is described a water-based pigment-containing ink for ink-jet printers which contains a polymeric dispersant having a backbone made up of hydrophilic parts and hydrophobic parts arranged regularly. In JP-B-4-23666 (the term "JP-B" as used herein means an "examined Japanese patent publication") is described a water-based pigment ink for drawing and the like which employ a hydrophilized pigment obtained by treating the surface voids of a pigment with an N-alkanolamine and adsorbing a nonionic surfactant onto the treated surface. These pigment dispersions containing a dispersant which has modified chemical properties or containing a pigment which has undergone a surface treatment or the like can have far higher stability than pigment dispersions prepared without using such a technique. However, the techniques described above each was intended mainly to stabilize dispersions of carbon black, which has relatively high polarity and is porous.
On the other hand, organic pigments are less apt to strongly adsorb dispersants because the particle surfaces thereof generally have low polarity. Techniques have been proposed in which a pigment derivative synthesized beforehand by forming parts having an affinity for a dispersant or incorporating polar groups is adsorbed onto the surface of a pigment and the adsorbed pigment derivative molecules are used as adsorption sites for the dispersant. Coating compositions or printing inks which utilize such a pigment derivative are described, e.g., in JP-B-58-28303, JP-A-5-98200, and JP-B-8-3049. However, since these techniques were mainly intended to produce non-aqueous pigment dispersions, it has been difficult to apply these techniques directly to aqueous dispersions.
The present inventors made intensive studies in order to obtain an aqueous organic-pigment dispersion having satisfactory pigment dispersibility and excellent storage stability. As a result, it has been found that an aqueous organic-pigment dispersion having the desired excellent pigment dispersibility and storage stability is obtained from an agglomerated organic pigment obtained by treating the surface of sulfonated agglomerates of an organic pigment with univalent counter ions to bring the agglomerate surface into a positively charged state. The present invention has been achieved based on this finding.